Let's start with a standard rod: a perfect cylinder designed to be printed vertically. If you're designing a rod, always make sure to chamfer the bottom of the cylinder so that you don't have to deal with any material deformation and you're able to get a nice clean base off of the part. This piece is viable and can be produced, and there aren't too many issues with it. In fact, it's kind of optimal because it's very easy to be ejected from the bed. However, the issue with printing a part like this is overall strength. It has slightly less strength than the z-axis, so if it's very long, it can actually be broken fairly easily. This is fine for reasonably high detail and fairly short parts. If it's too tall, you can start to get material shrinkage issues and even vibration issues at the very top of the part. So printing like this isn't always the optimal way to produce a very tall, skinny part.
If you're manufacturing something tall and skinny, you probably want to lay it sideways when printing. This way, when it's printed, the layer lines are in the plane of the rod itself, making it much stronger. But the issue with doing that is if you lay the rods sideways, you have a very small surface contact area with the bed, which needs some additional support by a brim. This can cause high rates of failure. While you want to print it on its side, you don't want a perfectly curved surface against the print bed. You want to ensure that it always has a decent amount of contact. To achieve this, you could flatten one side of the rod. Ensure that the tangent angle from wherever your cut is is at least 35 degrees to prevent sag on the outer side.
But this assumes that any rod or shaft you create has to be circular, which it doesn't. You can actually use hexagonal patterns, and these types of rods are fine. You can take it all the way up to almost a 10-sided profile where it will still print reliably, as long as the single bottom angle is not more than 35 degrees. This allows for a high level of contact area with the bed and a generally circular profile.
You can go vertical, which is okay, but once it gets tall, it can get a little finicky. Laying it sideways requires a flat surface, which means you either have to flatten the side of the rod or go for an overall hexagonal rod. There are many different designs for rods so we hope this gives a little more insight on the design process of these 3D printed rods.